Originally created by the Xalvorian overlords, the Fear Stalker has been adopted by many races throughout the galaxy, in spite of its dangerous nature. This hideous creature's primary advantage is not that it can sense fear in other living creatures, but that it also causes that fear. Pheromones and psychic emanations generated by Fearstalkers trigger the panic centres in organic brains, causing them to go haywire. Backed up by the prodigious strength of its arms and the savage ferocity of its many faces, this makes the Fearstalker a highly efficient hunter and killer of almost any organic creature.
This only makes it harder to control — for once a master has seen it in action, it is more likely to inspire fear, not just by its pheromones and psychic manipulations, but through the memories of the violence it has committed. Most of the hunters that employ Fearstalkers use emotion-dampening drugs when handling them. Some use robotic fear-suppression prosthetics. Eliminating fear is a necessity when controlling them, and even these tactics are not always enough. Controlling oneself through sheer willpower is certainly not an option. Once fear is eliminated, however, Fearstalkers can be quite docile, and take instruction easily.
The other ability the Fearstalker has that makes it such a fearsome tool is the ability to adapt to hostile environments. It does this so quickly that it can often be deployed on unfamiliar planets without warning. In moments it has acclimatized itself to burning gasses, corrosive winds, and radiation. Of course, the failure of the Xalvorians to conquer the slag miners of Illpath III can, in part, be laid at the feet of Fearstalkers eroded by the planet's insidious chemosphere, but many worlds that destroy unprotected human life do nothing to slow down the Fearstalker.